It is necessary in many electrical machines to provide an electrically conducting path between two members which are moving relative to one another. In dynamoelectric machines, for example, it is common to use a brush of electrically conducting material sliding on the surface of a slip-ring or commutator, to provide a current path between the rotor and an external connection. A principal requirement of such a brush is that it be able to carry a high current per unit area of interface between the brush and the surface which it contacts, and it should have high wear resistance, and low friction.
Solid carbon, graphite, and carbon-metal blocks have been used for brushes in the past. These blocks were limited to current densities of about 100 Amp./in..sup.2, for satisfactory operation in air. With such brushes, however, typically only about 1/10,000 of the brush face surface area is available as an actual interface contact for current transfer. This is due to irregular brush and slip-ring surface topography, oxide films present in the area of interface contact, and the accumulation of surface debris. High load forces, to improve brush contact, have resulted in high brush friction and wear.
McNab, in U.S. Pat. No. 3,668,451, attempted to remedy contact problems by using encased, multi-element brushes of silver or silver-copper, electro-plated or vacuum deposited on aluminum oxide or boron nitride non-conducting fibers. These brushes, coated with a simple all metal film, provided good contact surface area along with high strength and flexibility. They could be used for current densities on the order of about 1,000 Amp./in..sup.2, at continuous sliding speeds of up to about 18,000 ft./min. They presented heat and wear problems, however, and required disposing a lubricating material film such as molybdenum disulphide or a coating such as graphite or metal-graphite on the rotating rotor or on the slip-rings. This proved to be complicated and not completely effective.
With the last fifteen years, a large amount of interest has been shown in the development of homopolar machines for ship and vehicle propulsion and for inertial pulsed energy storage applications. Generally, these are machines in which the magnetic field and the current flowing in the active conductors maintain the same direction with respect to those conductors while the machine is in steady operation.
For high efficiency and acceptable machine size, the current collection systems for these high-current rotating machines must operate under very severe conditions. The current density levels at the brush contact interface may be as high as 5,000 Amp./in..sup.2 at continuous sliding speeds of up to 20,000 ft./min. Pulsed duty machinery may call for 25,000 Amp./in..sup.2 at 65,000 ft./min., at times, for hundreds of milliseconds.
Marshall, in U.S. Pat. No. 3,382,387, provided self-lubricating multi-element brushes, each element consisting of a copper or silver metal sheath containing weld inhibiting, conducting, lubricating powdered graphite. The graphite formed a conductive lubricating film, which while preventing metal sheath contact with the moving metal surface and thus an alloying effect, preserved direct electrical contact between the metal sheath and the moving metal surface. The brushes could be used for current densities on the order of 5,000 Amp./in..sup.2 at continuous sliding speeds of up to 33,000 ft./min. These type of brushes, however, tend to be somewhat inflexible, and filling the sheath, which has an inside diameter of about 0.02 inch, with graphite presents a fabrication problem. Also, at current densities over 5,000 Amp./in..sup.2 at 50,000 ft./min., the metal constituent must have unique lubrication, electrical power transfer, and heat dissipation requirements not yet met in the industry.
British Pat. No. 1,256,757 attempted to solve all current collection problems in homopolar dynamoelectric machines, by using a very sophisticated and costly liquid metal current collection system of the sodium-potassium type. While these metal type current collection systems need not worry about lubrication, and provide high electrical conductivity and intimacy of contact, they also pose serious machine, design, turbulence, toxicity and material compatibility problems.
In order for homopolar and other types of high-current rotating electrical machines to be economically attractive, new types of current collection means must be developed that are simple and inexpensive, and which keep frictional, electrical, and mechanical current collection losses at a minimum.